Restarting translated instructions

ABSTRACT

A processing system has a processor core ( 104 ) executing instructions of a first instruction set and an instruction translator ( 108 ) for generating translator output signals corresponding to one or more instructions of the first instruction set so as to emulate instructions of a second instruction set. The instruction translator ( 108 ) provides translator output signals specifying operations that are arranged so that the input variables to an instruction of the second instruction set are not changed until the final operation emulating that instruction is executed. An interrupt handler services an interrupt after execution of an operation of the instructions of the first instruction set. Arranging the translated sequences of instructions such that the input state is not altered until the final instruction is executed has the result that processing may be restarted after the interrupt either by rerunning the complete emulation if the final operation had not started when the interrupt occurred, or by running the next instruction from the second instruction set if the final operation had started when the interrupt occurred.

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] This invention relates to the field of data processing systems. More particularly, this invention relates to data processing systems having a processor core operable to execute instructions of a first instruction set and an instruction translator operable to translate instructions of a second instruction set into a form suitable for execution by the processor core.

2. Description of the Prior Art

[0002] It is known to provide instruction translators that may operate in conjunction with a processor core having a native instruction set to translate non-native instructions into native instructions for execution by the processor core. Whilst such an approach is attractive in extending the capabilities of a data processing system, it brings with it certain difficulties and complications.

[0003] One such problem is how to deal with interrupt signals. It is desirable that a processing system should respond to interrupt signals as rapidly as possible. This is particularly important in systems controlling real time operations. Interrupt latency can be a critical performance parameter and is measured using the worst case situation. Accordingly, when executing native instructions it is known to arrange that an interrupt signal will be responded to upon completion of the currently executing native instruction.

[0004] In the context of a system in which non-native instructions are translated into native instructions, it often arises that a single non-native instruction may be translated into more than one native instruction. Accordingly, if an interrupt is received during the execution of a sequence of native instructions representing a single non-native instruction, then the non-native instruction may be only partly have been completed and the state of the processing system may be uncertain. One way of dealing with this would be to provide additional hardware that was triggered upon receipt of an interrupt signal to store the current state of the processing system such that the state could be restored prior to restarting after the interrupt and so any partially completed non-native instruction would be able to be carried forward to completion. However, such an approach has the disadvantage of incurring an additional hardware overhead, significant additional complexity and may in itself degrade interrupt performance due to the need to save the state of the processing system prior to servicing the interrupt.

[0005] An alternative approach would be to control the system such that non-native instructions were treated as atomic, i.e. an interrupt would not be serviced until after a non-native instruction had fully completed its execution. This approach again adversely impacts interrupt latency.

[0006] Examples of known systems for translation between instruction sets and other background information may be found in the following U.S. Pat. Nos. 5,805,895; 3,955,180; 5,970,242; 5,619,665; 5,826,089; 5,925,123; 5,875,336; 5,937,193; 5,953,520; 6,021,469; 5,568,646; 5,758,115; 5,367,685; IBM Technical Disclosure Bulletin, March 1988, pp308-309, “System/370 Emulator Assist Processor For a Reduced Instruction Set Computer”; IBM Technical Disclosure Bulletin, July 1986, pp548-549, “Full Function Series/I Instruction Set Emulator”; IBM Technical Disclosure Bulletin, March 1994, pp605-606, “Real-Time CISC Architecture HW Emulator On A RISC Processor”; IBM Technical Disclosure Bulletin, March 1998, p272, “Performance Improvement Using An EMULATION Control Block”; IBM Technical Disclosure Bulletin, January 1995, pp537-540, “Fast Instruction Decode For Code Emulation on Reduced Instruction Set Computer/Cycles Systems”; IBM Technical Disclosure Bulletin, February 1993, pp23 1-234, “High Performance Dual Architecture Processor”; IBM Technical Disclosure Bulletin, August 1989, pp40-43, “System/370 I/O Channel Program Channel Command Word Prefetch”; IBM Technical Disclosure Bulletin, June 1985, pp305-306, “Fully Microcode-Controlled Emulation Architecture”; IBM Technical Disclosure Bulletin, March 1972, pp3074-3076, “Op Code and Status Handling For Emulation”; IBM Technical Disclosure Bulletin, August 1982, pp954-956, “On-Chip Microcoding of a Microprocessor With Most Frequently Used Instructions of Large System and Primitives Suitable for Coding Remaining Instructions”; IBM Technical Disclosure Bulletin, April 1983, pp5576-5577, “Emulation Instruction”; the book ARM System Architecture by S Furber; the book Computer Architecture: A Quantitative Approach by Hennessy and Patterson; and the book The Java Virtual Machine Specification by Tim Lindholm and Frank Yellin 1 ^(st) and 2 ^(nd) Editions.

[0007] The desirability of achieving low interrupt latency when executing non-native instructions is highlighted if one considers that one may wish to use such systems in real time applications, such as airbag control systems or anti-lock brake systems, in which the worst case interrupt latency may be a safety critical parameter.

SUMMARY OF THE INVENTION

[0008] Viewed from one aspect the present invention provides apparatus for processing data, said apparatus comprising:

[0009] (i) a processor core operable to execute operations as specified by instructions of a first instruction set;

[0010] (ii) an instruction translator operable to translate instructions of a second instruction set into translator output signals corresponding to instructions of said first instruction set, at least one instruction of said second instruction set specifying an operation to be executed using one or more input variables;

[0011] (iii) an interrupt handler responsive to an interrupt signal to interrupt execution of operations corresponding to instructions of said first instruction set after completion of execution of a currently executing operation; and

[0012] (iv) restart logic for restarting execution after said interrupt; wherein

[0013] (v) said instruction translator is operable to generate a sequence of one or more sets of translator output signals corresponding to instructions of said first instruction set to represent said at least one instruction of said second instruction set, each sequence being such that no change is made to said one or more input variables until a final operation within said sequence is executed; and

[0014] (vi) after occurrence of an interrupt during execution of a sequence of operations representing said at least one instruction of said second instruction set:

[0015] (a) if said interrupt occurred prior to starting execution of a final operation in said sequence, then said restart logic restarts execution at a first operation in said sequence; and

[0016] (b) if said interrupt occurred after starting execution of a final operation in said sequence, then said restart logic restarts execution at a next instruction following said sequence.

[0017] The invention allows for the translation of non-native instructions into a form that may take the equivalent of several native instructions to execute and yet provide interrupt servicing after completion of an operation corresponding to a native instruction without introducing undue difficulty on restarting. The invention achieves this by arranging that the translated sequence of operations does not make any changes to the input variables for that operation until the final operation is executed. Accordingly, if the interrupt occurred prior to the execution of the final operation, then the non-native instruction can be restarted in its entirety as the input variables will be unaltered whereas if the interrupt occurred after starting the execution of the final operation, then the final operation will complete and the restart logic can carry on from the next instruction following the non-native instruction during which the interrupt occurred.

[0018] It will be appreciated that the instructions from the second (non-native) instruction set may be fully translated into instructions of the first (native) instruction set. However, it is also possible that the instructions from the second instruction set may be translated into the form of control signals that are able to control the processor core in a similar manner to instructions from the first instruction set. A further possibility is that the instructions from the second instruction set may have capabilities beyond those of the instructions from the first instruction set and control signals derived from the instructions of the second instruction set may control the operation of the processor core in a manner that extends beyond the functions that may be provided by the instructions of the first instruction set.

[0019] Whilst it will be appreciated that the restart logic could be a dedicated hardware item, in preferred embodiments of the invention the restart logic may be part of the instruction translator. The instruction translator generates the translator output signals controlling the sequence of operations providing for the non-native instruction and so is readily able to determine whether or not the final operation had started when the interrupt occurred. This information is accordingly readily provided to restart logic within the instruction translator to determine whether the non-native instruction is restarted in its entirety or the next instruction is restarted.

[0020] A convenient way of keeping track of how the system should be restarted if an interrupt does occur is to store a pointer to a restart location with the pointer being advanced upon execution of the final operation. This pointer may conveniently be a program counter value pointing to a memory address of a memory location storing an instruction currently being translated.

[0021] Whilst the invention is applicable to many different types of instruction set, it is particularly useful when the second instruction set is one that specifies operations to be executed upon stack operands held in a stack. Such stack based systems typically read their input operands from the stack and write their output operands to the stack. When emulating such operation the present invention ensures that stack operands are not overwritten until after execution of the final operation has commenced. In a similar way, stack operands are not added to the stack until execution of the final operation has commenced.

[0022] It will be appreciated that the input variables of the system that control how a particular non-native instruction executes may extend beyond explicitly declared operands within that non-native instruction. In particular, surrounding system state variables may influence how a given instruction executes and accordingly the present invention provides that any such system state variables are not changed until execution of the final operation has commenced.

[0023] The need to ensure that system state is not changed until execution of the final operation may be a constraint upon the generation of efficient translated operation sequences. Thus, whilst interrupt latency may be preserved, the processing speed of non-native instructions may be impacted. However, this effect may be reduced in systems in which a register based processor core is emulating stack based instructions by providing that stack operands held within registers of the processor core are mapped to stack positions in accordance with a mapping state that is not updated until the final operation is executed such that input operands are not removed from the stack and output operands not added to the stack until the final operation without imposing too great a constraint upon the type of instructions that can be translated or the compactness of the translated sequences that may be achieved.

[0024] Viewed from another aspect the present invention provides a method of processing data, said method comprising the steps of:

[0025] (i) executing operations as specified by instructions of a first instruction set;

[0026] (ii) translating instructions of a second instruction set into translator output signals corresponding to instructions of said first instruction set, at least one instruction of said second instruction set specifying an operation to be executed using one or more input variables;

[0027] (iii) in response to an interrupt signal, interrupting execution of operations corresponding to instructions of said first instruction set after completion of execution of a currently executing operation; and

[0028] (iv) restarting execution after said interrupt; wherein

[0029] (v) said step of translating generates a sequence of one or more sets of translator output signals corresponding to instructions of said first instruction set to represent said at least one instruction of said second instruction set, each sequence being such that no change is made to said one or more input variables until a final operation within said sequence is executed; and

[0030] (vi) after occurrence of an interrupt during execution of a sequence of operations representing said at least one instruction of said second instruction set:

[0031] (a) if said interrupt occurred prior to starting execution of a final operation in said sequence, then restarting execution at a first operation in said sequence; and

[0032] (b) if said interrupt occurred after starting execution of a final operation in said sequence, then restarting execution at a next instruction following said sequence.

[0033] The invention also provides a computer program product bearing a computer program that can control a general purpose computer in accordance with the above techniques.

[0034] The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIGS. 1 and 2 schematically represent example instruction pipeline arrangements;

[0036]FIG. 3 illustrates in more detail a fetch stage arrangement;

[0037]FIG. 4 schematically illustrates the reading of variable length non-native instructions from within buffered instruction words within the fetch stage;

[0038]FIG. 5 schematically illustrates a data processing system for executing both processor core native instructions and instructions requiring translation;

[0039]FIG. 6 schematically illustrates, for a sequence of example instructions and states the contents of the registers used for stack operand storage, the mapping states and the relationship between instructions requiring translation and native instructions;

[0040]FIG. 7 schematically illustrates the execution of a non-native instruction as a sequence of native instructions;

[0041]FIG. 8 is a flow diagram illustrating the way in which the instruction translator may operate in a manner that preserves interrupt latency for translated instructions;.

[0042]FIG. 9 schematically illustrates the translation of Java bytecodes into ARM opcodes using hardware and software techniques;

[0043]FIG. 10 schematically illustrates the flow of control between a hardware based translator, a software based interpreter and software based scheduling;

[0044]FIGS. 11 and 12 illustrate another way of controlling scheduling operations using a timer based approach; and

[0045]FIG. 13 is a signal diagram illustrating the signals controlling the operation of the circuit of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046]FIG. 1 shows a first example instruction pipeline 30 of a type suitable for use in an ARM processor based system. The instruction pipeline 30 includes a fetch stage 32, a native instruction (ARM/Thumb instructions) decode stage 34, an execute stage 36, a memory access stage 38 and a write back stage 40. The execute stage 36, the memory access stage 38 and the write back stage 40 are substantially conventional. Downstream of the fetch stage 32, and upstream of the native instruction decode stage 34, there is provided an instruction translator stage 42. The instruction translator stage 42 is a finite state machine that translates Java bytecode instructions of a variable length into native ARM instructions. The instruction translator stage 42 is capable of multi-step operation whereby a single Java bytecode instruction may generate a sequence of ARM instructions that are fed along the remainder of the instruction pipeline 30 to perform the operation specified by the Java bytecode instruction. Simple Java bytecode instructions may required only a single ARM instruction to perform their operation, whereas more complicated Java bytecode instructions, or in circumstances where the surrounding system state so dictates, several ARM instructions may be needed to provide the operation specified by the Java bytecode instruction. This multi-step operation takes place downstream of the fetch stage 32 and accordingly power is not expended upon fetching multiple translated ARM instructions or Java bytecodes from a memory system. The Java bytecode instructions are stored within the memory system in a conventional manner such that additional constraints are not provided upon the memory system in order to support the Java bytecode translation operation.

[0047] As illustrated, the instruction translator stage 42 is provided with a bypass path. When not operating in an instruction translating mode, the instruction pipeline 30 may bypass the instruction translator stage 42 and operate in an essentially unaltered manner to provide decoding of native instructions.

[0048] In the instruction pipeline 30, the instruction translator stage 42 is illustrated as generating translator output signals that fully represent corresponding ARM instructions and are passed via a multiplexer to the native instruction decoder 34. The instruction translator 42 also generates some extra control signals that may be passed to the native instruction decoder 34. Bit space constraints within the native instruction encoding may impose limitations upon the range of operands that may be specified by native instructions. These limitations are not necessarily shared by the non-native instructions. Extra control signals are provided to pass additional instruction specifying signals derived from the non-native instructions that would not be possible to specify within native instructions stored within memory. As an example, a native instruction may only provide a relatively low number of bits for use as an immediate operand field within a native instruction, whereas the non-native instruction may allow an extended range and this can be exploited by using the extra control signals to pass the extended portion of the immediate operand to the native instruction decoder 34 outside of the translated native instruction that is also passed to the native instruction decoder 34.

[0049]FIG. 2 illustrates a further instruction pipeline 44. In this example, the system is provided with two native instruction decoders 46, 48 as well as a non-native instruction decoder 50. The non-native instruction decoder 50 is constrained in the operations it can specify by the execute stage 52, the memory stage 54 and the write back stage 56 that are provided to support the native instructions. Accordingly, the non-native instruction decoder 50 must effectively translate the non-native instructions into native operations (which may be a single native operation or a sequence of native operations) and then supply appropriate control signals to the execute stage 52 to carry out these one or more native operations. It will be appreciated that in this example the non-native instruction decoder does not produce signals that form a native instruction, but rather provides control signals that specify native instruction (or extended native instruction) operations. The control signals generated may not match the control signals generated by the native instruction decoders 46, 48.

[0050] In operation, an instruction fetched by the fetch stage 58 is selectively supplied to one of the instruction decoders 46, 48 or 50 in dependence upon the particular processing mode using the illustrated demultiplexer.

[0051]FIG. 3 schematically illustrates the fetch stage of an instruction pipeline in more detail. Fetching logic 60 fetches fixed length instruction words from a memory system and supplies these to an instruction word buffer 62. The instruction word buffer 62 is a swing buffer having two sides such that it may store both a current instruction word and a next instruction word. Whenever the current instruction word has been fully decoded and decoding has progressed onto the next instruction word, then the fetch logic 60 serves to replace the previous current instruction word with the next instruction word to be fetched from memory, i.e. each side of the swing buffer will increment by two in an interleaved fashion the instruction words that they successively store.

[0052] In the example illustrated, the maximum instruction length of a Java bytecode instruction is three bytes. Accordingly, three multiplexers are provided that enable any three neighbouring bytes within either side of the word buffer 62 to be selected and supplied to the instruction translator 64. The word buffer 62 and the instruction translator 64 are also provided with a bypass path 66 for use when native instructions are being fetched and decoded.

[0053] It will be seen that each instruction word is fetched from memory once and stored within the word buffer 62. A single instruction word may have multiple Java bytecodes read from it as the instruction translator 64 performs the translation of Java bytecodes into ARM instructions. Variable length translated sequences of native instructions may be generated without requiring multiple memory system reads and without consuming memory resource or imposing other constraints upon the memory system as the instruction translation operations are confined within the instruction pipeline.

[0054] A program counter value is associated with each Java bytecode currently being translated. This program counter value is passed along the stages of the pipeline such that each stage is able, if necessary, to use the information regarding the particular Java bytecode it is processing. The program counter value for a Java bytecode that translates into a sequence of a plurality of ARM instruction operations is not incremented until the final ARM instruction operation within that sequence starts to be executed. Keeping the program counter value in a manner that continues to directly point to the instruction within the memory that is being executed advantageously simplifies other aspects of the system, such as debugging and branch target calculation.

[0055]FIG. 4 schematically illustrates the reading of variable length Java bytecode instructions from the instruction buffer 62. At the first stage a Java bytecode instruction having a length of one is read and decoded. The next stage is a Java bytecode instruction that is three bytes in length and spans between two adjacent instruction words that have been fetched from the memory. Both of these instruction words are present within the instruction buffer 62 and so instruction decoding and processing is not delayed by this spanning of a variable length instruction between instruction words fetched. Once the three Java bytecodes have been read from the instruction buffer 62, the refill of the earlier fetched of the instruction words may commence as subsequent processing will continue with decoding of Java bytecodes from the following instruction word which is already present.

[0056] The final stage illustrated in FIG. 4 illustrates a second three bytecode instruction being read. This again spans between instruction words. If the preceding instruction word has not yet completed its refill, then reading of the instruction may be delayed by a pipeline stall until the appropriate instruction word has been stored into the instruction buffer 62. In some embodiments the timings may be such that the pipeline never stalls due to this type of behaviour. It will be appreciated that the particular example is a relatively infrequent occurrence as most Java bytecodes are shorter than the examples illustrated and accordingly two successive decodes that both span between instruction words is relatively uncommon. A valid signal may be associated with each of the instruction words within the instruction buffer 62 in a manner that is able to signal whether or not the instruction word has appropriately been refilled before a Java bytecode has been read from it.

[0057]FIG. 5 shows a data processing system 102 including a processor core 104 and a register bank 106. An instruction translator 108 is provided within the instruction path to translate Java Virtual Machine instructions to native ARM instructions (or control signals corresponding thereto) that may then be supplied to the processor core 104. The instruction translator 108 may be bypassed when native ARM instructions are being fetched from the addressable memory. The addressable memory may be a memory system such as a cache memory with further off-chip RAM memory. Providing the instruction translator 108 downstream of the memory system, and particularly the cache memory, allows efficient use to be made of the storage capacity of the memory system since dense instructions that require translation may be stored within the memory system and only expanded into native instructions immediately prior to being passed to the processor core 104.

[0058] The register bank 106 in this example contains sixteen general purpose 32-bit registers, of which four are allocated for use in storing stack operands, i.e. the set of registers for storing stack operands is registers R0, R1, R2 and R3.

[0059] The set of registers may be empty, partly filled with stack operands or completely filled with stack operands. The particular register that currently holds the top of stack operand may be any of the registers within the set of registers. It will thus be appreciated that the instruction translator may be in any one of seventeen different mapping states corresponding to one state when all of the registers are empty and four groups of four states each corresponding to a respective different number of stack operands being held within the set of registers and with a different register holding the top of stack operand. Table 1 illustrates the seventeen different states of the state mapping for the instruction translator 108. It will be appreciated that with a different number of registers allocated for stack operand storage, or as a result of constraints that a particular processor core may have in the way it can manipulate data values held within registers, the mapping states can very considerably depending upon the particular implementation and Table 1 is only given as an example of one particular implementation. TABLE 1 STATE 00000 R0 = EMPTY R1 = EMPTY R2 = EMPTY R3 = EMPTY STATE 00100 STATE 01000 STATE 01100 STATE 10000 R0 = TOS R0 = TOS R0 = TOS R0 = TOS R1 = EMPTY R1 = EMPTY R1 = EMPTY R1 = TOS-3 R2 = EMPTY R2 = EMPTY R2 = TOS-2 R2 = TOS-2 R3 = EMPTY R3 = TOS-1 R3 = TOS-1 R3 = TOS-1 STATE 00101 STATE 01001 STATE 01101 STATE 10001 R0 = EMPTY R0 = TOS-1 R0 = TOS-1 R0 = TOS-1 R1 = TOS R1 = TOS R1 = TOS R1 = TOS R2 = EMPTY R2 = EMPTY R2 = EMPTY R2 = TOS-3 R3 = EMPTY R3 = EMPTY R3 = TOS-2 R3 = TOS-2 STATE 00110 STATE 01010 STATE 01110 STATE 10010 R0 = EMPTY R0 = EMPTY R0 = TOS-2 R0 = TOS-2 R1 = EMPTY R1 = TOS-1 R1 = TOS-1 R1 = TOS-1 R2 = TOS R2 = TOS R2 = TOS R2 = TOS R3 = EMPTY R3 = EMPTY R3 = EMPTY R3 = TOS-3 STATE 00111 STATE 01011 STATE 01111 STATE 10011 R0 = EMPTY R0 = EMPTY R0 = EMPTY R0 = TOS-3 R1 = EMPTY R1 = EMPTY R1 = TOS-2 R1 = TOS-2 R2 = EMPTY R2 = TOS-1 R2 = TOS-1 R2 = TOS-1 R3 = TOS R3 = TOS R3 = TOS R3 = TOS

[0060] Within Table 1 it may be observed that the first three bits of the state value indicate the number of non-empty registers within the set of registers. The final two bits of the state value indicate the register number of the register holding the top of stack operand. In this way, the state value may be readily used to control the operation of a hardware translator or a software translator to take account of the currently occupancy of the set of registers and the current position of the top of stack operand.

[0061] As illustrated in FIG. 5 a stream of Java bytecodes J1, J2, J3 is fed to the instruction translator 108 from the addressable memory system. The instruction translator 108 then outputs a stream of ARM instructions (or equivalent control signals, possibly extended) dependent upon the input Java bytecodes and the instantaneous mapping state of the instruction translator 8, as well as other variables. The example illustrated shows Java bytecode J1 being mapped to ARM instructions A¹ 1 and A¹ 2. Java bytecode J2 maps to ARM instructions A² 1, A² 2 and A² 3. Finally, Java bytecode J3 maps to ARM instruction A³ 1. Each of the Java bytecodes may require one or more stack operands as inputs and may produce one or more stack operands as an output. Given that the processor core 104 in this example is an ARM processor core having a load/store architecture whereby only data values held within registers may be manipulated, the instruction translator 108 is arranged to generate ARM instructions that, as necessary, fetch any required stack operands into the set of registers before they are manipulated or store to addressable memory any currently held stack operands within the set of registers to make room for result stack operands that may be generated. It will be appreciated that each Java bytecode may be considered as having an associated “require full” value indicating the number of stack operands that must be present within the set of registers prior to its execution together with a “require empty” value indicating the number of empty registers within the set of registers that must be available prior to execution of the ARM instructions representing the Java opcode.

[0062] Table 2 illustrates the relationship between initial mapping state values, require full values, final state values and associated ARM instructions. The initial state values and the final state values correspond to the mapping states illustrated in Table 1. The instruction translator 108 determines a require full value associated with the particular Java bytecode (opcode) it is translating. The instruction translator (108), in dependence upon the initial mapping state that it has, determines whether or not more stack operands need to be loaded into the set of registers prior to executing the Java bytecode. Table 1shows the initial states together with tests applied to the require full value of the Java bytecode that are together applied to determine whether a stack operand needs to be loaded into the set of registers using an associated ARM instruction (an LDR instruction) as well as the final mapping state that will be adopted after such a stack cache load operation. In practice, if more than one stack operand needs to be loaded into the set of registers prior to execution of the Java bytecode, then multiple mapping state transitions will occur, each with an associated ARM instruction loading a stack operand into one of the registers of the set of registers. In different embodiments it may be possible to load multiple stack operands in a single state transition and accordingly make mapping state changes beyond those illustrated in Table 2. TABLE 2 INITIAL REQUIRE FINAL STATE FULL STATE ACTIONS 00000 >0 00100 LDR R0, [Rstack, #-4]! 00100 >1 01000 LDR R3, [Rstack, #-4]! 01001 >2 01101 LDR R3, [Rstack, #-4]! 01110 >3 10010 LDR R3, [Rstack, #-4]! 01111 >3 10011 LDR R0, [Rstack, #-4]! 01100 >3 10000 LDR R1, [Rstack, #-4]! 01101 >3 10001 LDR R2, [Rstack, #-4]! 01010 >2 01110 LDR R0, [Rstack, #-4]! 01011 >2 01111 LDR R1, [Rstack, #-4]! 01000 >2 01100 LDR R2, [Rstack, #-4]! 00110 >1 01010 LDR R1, [Rstack, #-4]! 00111 >1 01011 LDR R2, [Rstack, #-4]! 00101 >1 01001 LDR R0, [Rstack, #-4]!

[0063] As will be seen from Table 2, a new stack operand loaded into the set of registers storing stack operands will form a new top of stack operand and this will be loaded into a particular one of the registers within the set of registers depending upon the initial state.

[0064] Table 3 in a similar manner illustrates the relationship between initial state, require empty value, final state and an associated ARM instruction for emptying a register within the set of registers to move between the initial state and the final state if the require empty value of a particular Java bytecode indicates that it is necessary given the initial state before the Java bytecode is executed. The particular register values stored off to the addressable memory with an STR instruction will vary depending upon which of the registers is the current top of stack operand. TABLE 3 INITIAL REQUIRE FINAL STATE EMPTY STATE ACTIONS 00100 >3 00000 STR R0, [Rstack], #4 01001 >2 00101 STR R0, [Rstack], #4 01110 >1 01010 STR R0, [Rstack], #4 10011 >0 01111 STR R0, [Rstack], #4 10000 >0 01100 STR R1, [Rstack], #4 10001 >0 01101 STR R2, [Rstack], #4 10010 >0 01110 STR R3, [Rstack], #4 01111 >1 01011 STR R1, [Rstack], #4 01100 >1 01000 STR R2, [Rstack], #4 01101 >1 01001 STR R3, [Rstack], #4 01010 >2 00110 STR R1, [Rstack], #4 01011 >2 00111 STR R2, [Rstack], #4 01000 >2 00100 STR R3, [Rstack], #4 00110 >3 00000 STR R2, [Rstack], #4 00111 >3 00000 STR R3, [Rstack], #4 00101 >3 00000 STR R1, [Rstack], #4

[0065] It will be appreciated that in the above described example system the require full and require empty conditions are mutually exclusive, that is to say only one of the require full or require empty conditions can be true at any given time for a particular Java bytecode which the instruction translator is attempting to translate. The instruction templates used by the instruction translator 108 together with the instructions it is chosen to support with the hardware instruction translator 108 are selected such that this mutually exclusive requirement may be met. If this requirement were not in place, then the situation could arise in which a particular Java bytecode required a number of input stack operands to be present within the set of registers that would not allow sufficient empty registers to be available after execution of the instruction representing the Java bytecode to allow the results of the execution to be held within the registers as required.

[0066] It will be appreciated that a given Java bytecode will have an overall nett stack action representing the balance between the number of stack operands consumed and the number of stack operands generated upon execution of that Java bytecode. Since the number of stack operands consumed is a requirement prior to execution and the number of stack operands generated is a requirement after execution, the require full and require empty values associated with each Java bytecode must be satisfied prior to execution of that bytecode even if the nett overall action would in itself be met. Table 4 illustrates the relationship between an initial state, an overall stack action, a final state and a change in register use and relative position of the top of stack operand (TOS). It may be that one or more of the state transitions illustrated in Table 2 or Table 3 need to be carried out prior to carrying out the state transitions illustrated in Table 4 in order to establish the preconditions for a given Java bytecode depending on the require full and require empty values of the Java bytecode. TABLE 4 INITIAL STACK FINAL STATE ACTION STATE ACTIONS 00000 +1 00101 R1 <- TOS 00000 +2 01010 R1 <- TOS-1, R2 <- TOS 00000 +3 01111 R1 <- TOS-2, R2 <- TOS-1, R3 <- TOS 00000 +4 10000 R0 <- TOS, R1 <- TOS-3, R2 <- TOS-2, R3 <- TOS-1 00100 +1 01001 R1 <- TOS 00100 +2 01110 R1 <- TOS-1, R2 <- TOS 00100 +3 10011 R1 <- TOS-2, R2 <- TOS-1, R3 <- TOS 00100 −1 00000 R0 <- EMPTY 01001 +1 01110 R2 <- TOS 01001 +2 10011 R2 <- TOS-1, R3 <- TOS 01001 −1 00100 R1 <- EMPTY 01001 −2 00000 R0 <- EMPTY, R1 <- EMPTY 01110 +1 10011 R3 <- TOS 01110 −1 01001 R2 <- EMPTY 01110 −2 00100 R1 <- EMPTY, R2 <- EMPTY 01110 −3 00000 R0 <- EMPTY, R1 <- EMPTY, R2 <- EMPTY 10011 −1 01110 R3 <- EMPTY 10011 −2 01001 R2 <- EMPTY, R3 <- EMPTY 10011 −3 00100 R1 <- EMPTY, R2 <- EMPTY, R3 <- EMPTY 10011 −4 00000 R0 <- EMPTY, R1 <- EMPTY, R2 <- EMPTY, R3 <- EMPTY 10000 −1 01111 R0 <- EMPTY 10000 −2 01010 R0 <- EMPTY, R3 <- EMPTY 10000 −3 00101 R0 <- EMPTY, R2 <- EMPTY, R3 <- EMPTY 10000 −4 00000 R0 <- EMPTY, R1 <- EMPTY, R2 <- EMPTY, R3 <- EMPTY 10001 −1 01100 R1 <- EMPTY 10001 −2 01011 R0 <- EMPTY, R1 <- EMPTY 10001 −3 00110 R0 <- EMPTY, R1 <- EMPTY, R3 <- EMPTY 10001 −4 00000 R0 <- EMPTY, R1 <- EMPTY, R2 <- EMPTY, R3 <- EMPTY 10010 −1 01101 R2 <- EMPTY 10010 −2 01000 R1 <- EMPTY, R2 <- EMPTY 10010 −3 00111 R0 <- EMPTY, R1 <- EMPTY, R2 <- EMPTY 10010 −4 00000 R0 <- EMPTY, R1 <- EMPTY, R2 <- EMPTY, R3 <- EMPTY 01111 +1 10000 R0 <- TOS 01111 −1 01010 R3 <- EMPTY 01111 −2 00101 R2 <- EMPTY, R3 <- EMPTY 01111 −3 00000 R1 <- EMPTY, R2 <- EMPTY, R3 <- EMPTY 01100 +1 10001 R1 <- TOS 01100 −1 01011 R0 <- EMPTY 01100 −2 00110 R0 <- EMPTY, R3 <- EMPTY 01100 −3 00000 R0 <- EMPTY, R2 <- EMPTY, R3 <- EMPTY 01101 +1 10010 R2 <- TOS 01101 −1 01000 R1 <- EMPTY 01101 −2 00111 R0 <- EMPTY, R1 <- EMPTY 01101 −3 00000 R0 <- EMPTY, R1 <- EMPTY, R3 <- EMPTY 01010 +1 01111 R3 <- TOS 01010 +2 10000 R3 <- TOS-1, R0 <- TOS 01010 −1 00101 R2 <- EMPTY 01010 −2 00000 R1 <- EMPTY, R2 <- EMPTY 01011 +1 01100 R0 <- TOS 01011 +2 10001 R0 <- TOS-1, R1 <- TOS 01011 −1 00110 R3 <- EMPTY 01011 −2 00000 R2 <- EMPTY, R3 <- EMPTY 01000 +1 01101 R1 <- TOS 01000 +2 10010 R1 <- TOS-1, R2 <- TOS 01000 −1 00111 R0 <- EMPTY 01000 −2 00000 R0 <- EMPTY, R3 <- EMPTY 00110 +1 01011 R3 <- TOS 00110 +2 01100 R0 <- TOS, R3 <- TOS-1 00110 +3 10001 R1 <- TOS, R0 <- TOS-1, R3 <- TOS-2 00110 −1 00000 R2 <- EMPTY 00111 +1 01000 R0 <- TOS 00111 +2 01101 R0 <- TOS-1, R1 <- TOS 00111 +3 10010 R0 <- TOS-2, R1 <- TOS-1, R2 <- TOS 00111 −1 00000 R3 <- EMPTY 00101 +1 01010 R2 <- TOS 00101 +2 01111 R2 <- TOS-1, R3 <- TOS 00101 +3 10000 R2 <- TOS-2, R3 <- TOS-1, R1 <- TOS 00101 −1 00000 R1 <- EMPTY

[0067] It will be appreciated that the relationships between states and conditions illustrated in Table 2, Table 3 and Table 4 could be combined into a single state transition table or state diagram, but they have been shown separately above to aid clarity.

[0068] The relationships between the different states, conditions, and nett actions may be used to define a hardware state machine (in the form of a finite state machine) for controlling this aspect of the operation of the instruction translator 108. Alternatively, these relationships could be modelled by software or a combination of hardware and software.

[0069] There follows below an example of a subset of the possible Java bytecodes that indicates for each Java bytecode of the subset the associated require full, require empty and stack action values for that bytecode which may be used in conjunction with Tables 2, 3 and 4. --- iconst_0 Operation: Push int constant Stack: . . . => . . . , 0 Require-Full = 0 Require-Empty = 1 Stack-Action = +1 --- iadd Operation: Add mt Stack: . . . , value1, value2 => result Require-Full = 2 Require-Empty = 0 Stack-Action = −1 --- lload_0 Operation: Load long from local variable Stack: . . . => . . . , value.word1, value.word2 Require-Full = 0 Require-Empty = 2 Stack-Action = +2 --- lastore Operation: Store into long array Stack: . . . , arrayref, index, value.word1, value.word2 => . . . Require-Full = 4 Require-Empty = 0 Stack-Action = −4 --- land Operation Boolean AND long Stack: . . . , value1.word1, value1.word2, value2.word1, value2.word2 => . . . result.word1, result.word2 Require-Full = 4 Require-Empty = 0 Stack-Action-2 --- iastore Operation: Store into mt array Stack: . . . , arrayref, index, value => . . . Require-Full = 3 Require-Empty = 0 Stack-Action = −3 --- ineg Operation: Negate int Stack: . . . , value => . . . , result Require-Full = 1 Require-Empty = 0 Stack-Action = 0

[0070] There also follows example instruction templates for each of the Java bytecode instructions set out above. The instructions shown are the ARM instructions which implement the required behaviour of each of the Java bytecodes. The register field “TOS-3”, “TOS-2”, “TOS-1”, “TOS”, “TOS+1” and “TOS+2” may be replaced with the appropriate register specifier as read from Table 1 depending upon the mapping state currently adopted. The denotation “TOS+n” indicates the Nth register above the register currently storing the top of stack operand starting from the register storing the top of stack operand and counting upwards in register value until reaching the end of the set of registers at which point a wrap is made to the first register within the set of registers. iconst_0 MOV tos + 1, #0 lload_0 LDR tos + 2, [vars, #4] LDR tos + 1, [vars, #0] iastore LDR Rtmp2, [tos-2, #4] LDR Rtmp1, [tos-2, #0] CMP tos-1, Rtmp2, LSR #5 BLXCS Rexc STR tos, [Rtmp1, tos-1, LSL #2] lastore LDR Rtmp2, [tos-3, #4] LDR Rtmp1, [tos-3, #0] CMP tos-2, Rtmp2, LSR #5 BLXCS Rexc STR tos-1, [Rtmp1, tos-2, LSL #3]! STR tos, [Rtmp1, #4] iadd ADD tos-1, tos-1, tos ineg RSB tos, tos, #0 land AND tos-2, tos-2, tos AND tos-3, tos-3, tos-1

[0071] An example execution sequence is illustrated below of a single Java bytecode executed by a hardware translation unit 108 in accordance with the techniques described above. The execution sequence is shown in terms of an initial state progressing through a sequence of states dependent upon the instructions being executed, generating a sequence of ARM instructions as a result of the actions being performed on each state transition, the whole having the effect of translating a Java bytecode to a sequence of ARM instructions. Initial state: 00000 Instruction: iadd (Require-Full = 2, Require-Empty = 0, Stack-Action = −1) Condition: Require-Full > 0 State Transition: 00000 > 0 00100 ARM Instruction(s) LDR R0, [Rstack, #−4]! Next state: 00100 Instruction: iadd (Require-Full = 2, Require-Empty = 0, Stack-Action = −1) Condition: Requite-Full > 1 State Transition: 00100 > 1 01000 ARM Instructions(s): LDR R3, [Rstack, #-4]! Next state: 01000 Instruction: iadd (Require-Full = 2, Require-Empty = 0, Stack-Action = −1) Condition: Stack-Action = −1 State Transition: 01000 −1 00111 Instruction template: ADD tos-1, tos-1, tos ARM Instructions (5) (after substitution): ADD R3, R3, R0 Next state: 00111

[0072]FIG. 6 illustrates in a different way the execution of a number of further Java bytecode instructions. The top portion of FIG. 6 illustrates the sequence of ARM instructions and changes of mapping states and register contents that occur upon execution of an iadd Java bytecode instruction. The initial mapping state is 00000 corresponding to all of the registers within the set of registers being empty. The first two ARM instructions generated serve to POP two stack operands into the registers storing stack operands with the top of stack “TOS” register being R0. The third ARM instruction actually performs the add operation and writes the result into register R3 (which now becomes the top of stack operand) whilst consuming the stack operand that was previously held within register R1, thus producing an overall stack action of −1.

[0073] Processing then proceeds to execution of two Java bytecodes each representing a long load of two stack operands. The require empty condition of 2 for the first Java bytecode is immediately met and accordingly two ARM LDR instructions may be issued and executed. The mapping state after execution of the first long load Java bytecode is 01101. In this state the set of registers contains only a single empty register. The next Java bytecode long load instruction has a require empty value of 2 that is not met and accordingly the first action required is a PUSH of a stack operand to the addressable memory using an ARM STR instruction. This frees up a register within the set of registers for use by a new stack operand which may then be loaded as part of the two following LDR instructions. As previously mentioned, the instruction translation may be achieved by hardware, software, or a combination of the two. Given below is a subsection of an example software interpreter generated in accordance with the above described techniques. Interpret LDRB Rtmp, [Rjpc, #1]! LDR pc, [pc, Rtmp, lsl #2] DCD 0 . . . DCD do_iconst_0; Opcode 0 × 03 . . . DCD do_lload_0; Opcode 0 × 1e . . . DCD do_iastore; Opcode 0 × 4f DCD do_lastore; Opcode 0 × 50 . . . DCD do_iadd; Opcode 0 × 60 . . . DCD do_ineg; Opcode 0 × 74 . . . DCD do_land; Opcode 0 × 7f . . . do_iconst_0 MOV R0, #0 STR R0, [Rstack], #4 B Interpret do_lload_0 LDMIA Rvars, {R0, R1} STMIA Rstack!, {R0, R1} B Interpret do_iastore LDMDB Rstack!, {R0, R1, R2} LDR Rtmp2, [r0, #4] LDR Rtmp1, [r0, #0] CMP R1, Rtmp2, LSR #5 BCS ArrayBoundException STR R2, [Rtmp1, R1, LSL #2] B Interpret do_lastore LDMDB Rstack!, {R0, R1, R2, R3} LDR Rtmp2, [r0, #4) LDR Rtmp1, [r0, #0) CMP R1, Rtmp2, LSR #5 BCS ArrayBoundException STR R2, [Rtmp1, R1, LSL #3]! STR R3, [Rtmp1, #4] B Interpret do_iadd LDMDB Rstack!, {r0, r1} ADD r0, r0, r1 STR r0, [Rstack], #4 B Interpret do_ineg LDR r0, [Rstack, #-4]! RSB tos, tos, #0 STR r0, [Rstack], #4 B Interpret do_land LDMDB Rstack!, {r0, r1, r2, r3} AND r1, r1, r3 AND r0, r0, r2 STMIA Rstack!, {r0, r1} B Interpret State_00000_Interpret LDRB Rtmp, [Rjpc, #1]! LDR pc, [pc, Rtmp, lsl #2] DCD 0 . . . DCD State_00000_do_iconst_0; Opcode 0 × 03 . . . DCD State_00000_do_lload_0; Opcode 0 × 1e . . . DCD State_00000_do_iastore; Opcode 0 × 4f DCD State_00000_do_lastore; Opcode 0 × 50 . . . DCD State_00000_do_iadd; Opcode 0 × 60 . . . DCD State_00000_do_ineg; Opcode 0 × 74 . . . DCD State_00000_do_land; Opcode 0 × 7f . . . State_00000_do_iconst_0 MOV R1, #0 B State_00101_Interpret State_00000_do_lload_0 LDMIA Rvars, {R1, R2} B State_01010_Interpret State_00000_do_iastore LDMDB Rstack!, {R0, R1, R2} LDR Rtmp2, [r0, #4] LDR Rtmp1, [r0, #0] CMP R1, Rtmp2, LSR #5 BCS ArrayBoundException STR R2, [Rtmp1, R1, LSL #2] B State_00000_Interpret State_00000_do_lastore LDMDB Rstack!, {R0, R1, R2, R3} LDR Rtmp2, [r0, #4] LDR Rtmp1, [r0, #0] CMP R1, Rtmp2, LSR #5 BCS ArrayBoundException STR R2, [Rtmp1, R1, LSL #3]! STR R3, [Rtmp1, #4] B State_00000_Interpret State_00000_do_iadd LDMDB Rstack!, {R1, R2} ADD r1, r1, r2 B State_00101_Interpret State_00000_do_ineg LDR r1, [Rstack, #-4]! RSB r1, r1, #0 B State_00101_Interpret State_00000_do_land LDR r0, [Rstack, #-4]! LDMDB Rstack!, {r1, r2, r3} AND r2, r2, r0 AND r1, r1, r3 B State_01010_Interpret State_00100_Interpret LDRB Rtmp, [Rjpc, #1]! LDR pc, [pc, Rtmp, lsl #2] DCD 0 . . . DCD State_00100_do_iconst_0; Opcode 0 × 03 . . . DCD State_00100_do_lload_0; Opcode 0 × 1e . . . DCD State_00100_do_iastore; Opcode 0 × 4f DCD State_00100_do_lastore; Opcode 0 × 50 . . . DCD State_00100_do_iadd; Opcode 0 × 60 . . . DCD State_00100_do_ineg; Opcode 0 × 74 . . . DCD State_00100_do_land; Opcode 0 × 7f . . . State_00100_do_iconst_0 MOV R1, #0 B State_01001_Interpret State_00100_do_lload_0 LDMIA Rvars, {r1, R2} B State_01110_Interpret State_00100_do_iastore LDMDB Rstack!, {r2, r3} LDR Rtmp2, [r2, #4] LDR Rtmp1, [r2, #0] CMP R3, Rtmp2, LSR #5 BCS ArrayBoundException STR R0, [Rtmp1, R3, lsl #2] B State_00000_Interpret State_00100_do_lastore LDMDB Rstack!, {r1, r2, r3} LDR Rtmp2, [r1, #4] LDR Rtmp1, [r1, #0] CMP r2, Rtmp2, LSR #5 BCS ArrayBoundException STR r3, [Rtmp1, r2, lsl #3]! STR r0, [Rtmp1, #4] B State_00000_Interpret State_00100_do_iadd LDR r3, [Rstack, #-4]! ADD r3, r3, r0 B State_00111_Interpret State_00100_do_ineg RSB r0, r0, #0 B State_00100_Interpret State_00100_do_land LDMDB Rstack!, {r1, r2, r3} AND r2, r2, r0 AND r1, r1, r3 B State_01010_Interpret State_01000_Interpret LDRB Rtmp, [Rjpc, #1]! LDR pc, [pc, Rtmp, lsl #2] DCD 0 . . . DCD State_01000_do_iconst_0; Opcode 0 × 03 . . . DCD State_01000_do_lload_0; Opcode 0 × 1e . . . DCD State_01000_do_iastore; Opcode 0 × 4f DCD State_01000_do_lastore; Opcode 0 × 50 . . . DCD State_01000_do_iadd; Opcode 0 × 60 . . . DCD State_01000_do_ineg; Opcode 0 × 74 . . . DCD State_01000_do_land; Opcode 0 × 7f . . . State_01000_do_iconst_0 MOV R1, #0 B State_01101_Interpret State_01000_do_lload_0 LDMIA Rvars, {r1, r2} B State_10010_Interpret State_01000_do_iastore LDR r1, [Rstack, #-4]! LDR Rtmp2, [R3, #4] LDR Rtmp1, [R3, #0] CMP r0, Rtmp2, LSR #5 BCS ArrayBoundException STR r1, [Rtmp1, r0, lsl #2] B State_00000_Interpret State 01000_do_lastore LDMDB Rstack!, {r1, r2} LDR Rtmp2, {r3, #4} LDR Rtmp1, {R3, #0} CMP r0, Rtmp2, LSR #5 BCS ArrayBoundException STR r1, [Rtmp1, r0, lsl #3]! STR r2, [Rtmp1, #4] B State_00000_Interpret State_01000_do_iadd ADD r3, r3, r0 B State_00111_Interpret State_01000_do_ineg RSB r0, r0, #0 B State_01000_Interpret State_01000_do_land LDMDB Rstack!, {r1, r2} AND R0, R0, R2 AND R3, R3, R1 B State_01000_Interpret State_01100_Interpret . . . State_10000_Interpret . . . State_00101_Interpret . . . State_01001_Interpret . . . State_01101_Interpret . . . State_10001_Interpret . . . State_00110_Interpret . . . State_01010_Interpret . . . State_01110_Interpret . . . State_10010_Interpret . . . State_00111_Interpret . . . State_01011_Interpret . . . State_01111_Interpret . . . State_10011_Interpret . . .

[0074]FIG. 7 illustrates a Java bytecode instruction “laload” which has the function of reading two words of data from within a data array specified by two words of data starting at the top of stack position. The two words read from the data array then replace the two words that specified their position and to form the topmost stack entries.

[0075] In order that the “laload” instruction has sufficient register space for the temporary storage of the stack operands being fetched from the array without overwriting the input stack operands that specify the array and position within the array of the data, the Java bytecode instruction is specified as having a require empty value of 2, i.e. two of the registers within the register bank dedicated to stack operand storage must be emptied prior to executing the ARM instructions emulating the “laload” instruction. If there are not two empty registers when this Java bytecode is encountered, then store operations (STRs) may be performed to PUSH stack operands currently held within the registers out to memory so as to make space for the temporary storage necessary and meet the require empty value for the instruction.

[0076] The instruction also has a require full value of 2 as the position of the data is specified by an array location and an index within that array as two separate stack operands. The drawing illustrates the first state as already meeting the require full and require empty conditions and having a mapping state of “01001”. The “laload” instruction is broken down into three ARM instructions. The first of these loads the array reference into a spare working register outside of the set of registers acting as a register cache of stack operands. The second instruction then uses this array reference in conjunction with an index value within the array to access a first array word that is written into one of the empty registers dedicated to stack operand storage.

[0077] It is significant to note that after the execution of the first two ARM instructions, the mapping state of the system is not changed and the top of stack pointer remains where it started with the registers specified as empty still being so specified.

[0078] The final instruction within the sequence of ARM instructions loads the second array word into the set of registers for storing stack operands. As this is the final instruction, if an interrupt does occur during it, then it will not be serviced until after the instruction completes and so it is safe to change the input state with this instruction by a change to the mapping state of the registers storing stack operands. In this example, the mapping state changes to “01011”which places the new top of stack pointer at the second array word and indicates that the input variables of the array reference and index value are now empty registers, i.e. marking the registers as empty is equivalent to removing the values they held from the stack.

[0079] It will be noted that whilst the overall stack action of the “laload” instruction has not changed the number of stack operands held within the registers, a mapping state swap has nevertheless occurred. The change of mapping state performed upon execution of the final operation is hardwired into the instruction translator as a function of the Java bytecode being translated and is indicated by the “swap” parameter shown as a characteristic of the “laload”instruction.

[0080] Whilst the example of this drawing is one specific instruction, it will be appreciated that the principles set out may be extended to many different Java bytecode instructions that are emulated as ARM instructions or other types of instruction.

[0081]FIG. 8 is a flow diagram schematically illustrating the above technique. At step 10 a Java bytecode is fetched from memory. At step 12 the require full and require empty values for that Java bytecode are examined. If either of the require empty or require full conditions are not met, then respective PUSH and POP operations of stack operands (possibly multiple stack operands) may be performed with steps 14 and 16. It is will be noted that this particular system does not allow the require empty and require full conditions to be simultaneously unmet. Multiple passes through steps 14 and 16 may be required until the condition of step 12 is met.

[0082] At step 18, the first ARM instruction specified within the translation template for the Java bytecode concerned is selected. At step 20, a check is made as to whether or not the selected ARM instruction is the final instruction to be executed in the emulation of the Java bytecode fetched at step 10. If the ARM instruction being executed is the final instruction, then step 21 serves to update the program counter value to point to the next Java bytecode in the sequence of instructions to be executed. It will be understood that if the ARM instruction is the final instruction, then it will complete its execution irrespective of whether or not an interrupt now occurs and accordingly it is safe to update the program counter value to the next Java bytecode and restart execution from that point as the state of the system will have reached that matching normal, uninterrupted, full execution of the Java bytecode. If the test at step 20 indicates that the final bytecode has not been reached, then updating of the program counter value is bypassed.

[0083] Step 22 executes the current ARM instruction. At step 24 a test is made as to whether or not there are any more ARM instructions that require executing as part of the template. If there are more ARM instructions, then the next of these is selected at step 26 and processing is returned to step 20. If there are no more instructions, then processing proceeds to step 28 at which any mapping change/swap specified for the Java bytecode concerned is performed in order to reflect the desired top of stack location and full/empty status of the various registers holding stack operands.

[0084]FIG. 8 also schematically illustrates the points at which an interrupt if asserted is serviced and then processing restarted after an interrupt. An interrupt starts to be serviced after the execution of an ARM instruction currently in progress at step 22 with whatever is the the current ARM instruction executing is the final instruction within the template sequence, then step 21 will have just updated the program counter value and accordingly this will point to the next Java bytecode (or ARM instruction should an instruction set switch have just been initiated). If the currently executing ARM instruction is anything other than the final instruction in the sequence, then the program counter value will still be the same as that indicated at the start of the execution of the Java bytecode concerned and accordingly when a return is made, the whole Java bytecode will be re-executed.

[0085]FIG. 9 illustrates a Java bytecode translation unit 68 that receives a stream of Java bytecodes and outputs a translated stream of ARM instructions (or corresponding control signals) to control the action of a processor core. As described previously, the Java bytecode translator 68 translates simple Java bytecodes using instruction templates into ARM instructions or sequences of ARM instructions. When each Java bytecode has been executed, then a counter value within scheduling control logic 70 is decremented. When this counter value reaches 0, then the Java bytecode translation unit 68 issues an ARM instruction branching to scheduling code that manages scheduling between threads or tasks as appropriate.

[0086] Wilst simple Java bytecodes are handled by the Java bytecode translation unit 68 itself providing high speed hardware based execution of these bytecodes, bytecodes requiring more complex processing operations are sent to a software interpreter provided in the form of a collection of interpretation routines (examples of a selection of such routines are given earlier in this description). More specifically, the Java bytecode translation unit 68 can determined that the bytecode it has received is not one which is supported by hardware translation and accordingly a branch can be made to an address dependent upon that Java bytecode where a software routine for interpreting that bytecode is found or referenced. This mechanism can also be employed when the scheduling logic 70 indicates that a scheduling operation is needed to yield a branch to the scheduling code.

[0087]FIG. 10 illustrates the operation of the embodiment of FIG. 9 in more detail and the split of tasks between hardware and software. All Java bytecodes are received by the Java bytecode translation unit 68 and cause the counter to be decremented at step 72. At step 74 a check is made as to whether or not the counter value has reached 0. If the counter value has reached 0 (counting down from either a predetermined value hardwired into the system or a value that may be user controlled/programmed), then a branch is made to scheduling code at step 76. Once the scheduling code has completed at step 76, control is returned to the hardware and processing proceeds to step 72, where the next Java bytecode is fetched and the counter again decremented. Since the counter reached 0, then it will now roll round to a new, non-zero value. Alternatively, a new value may be forced into the counter as part of the exiting of the scheduling process at step 76.

[0088] If the test at step 74 indicated that the counter did not equal 0, then step 78 fetches the Java bytecode. At step 80 a determination is made as to whether the fetched bytecode is a simple bytecode that may be executed by hardware translation at step 82 or requires more complex processing and accordingly should be passed out for software interpretation at step 84. If processing is passed out to software interpretation, then once this has completed control is returned to the hardware where step 72 decrements the counter again to take account of the fetching of the next Java bytecode.

[0089]FIG. 11 illustrates an alternative control arrangement. At the start of processing at step 86 an instruction signal (scheduling signal) is deasserted. At step 88, a fetched Java bytecode is examined to see if it is a simple bytecode for which hardware translation is supported. If hardware translation is not supported, then control is passed out to the interpreting software at step 90 which then executes a ARM instruction routine to interpret the Java bytecode. If the bytecode is a simple one for which hardware translation is supported, then processing proceeds to step 92 at which one or more ARM instructions are issued in sequence by the Java bytecode translation unit 68 acting as a form of multi-cycle finite state machine. Once the Java bytecode has been properly executed either at step 90 or at step 92, then processing proceeds to step 94 at which the instruction signal is asserted for a short period prior to being deasserted at step 86. The assertion of the instruction signal indicates to external circuitry that an appropriate safe point has been reached at which a timer based scheduling interrupt could take place without risking a loss of data integrity due to the partial execution of an interpreted or translated instruction.

[0090]FIG. 12 illustrates example circuitry that may be used to respond to the instruction signal generated in FIG. 11. A timer 96 periodically generates a timer signal after expiry of a given time period. This timer signal is stored within a latch 98 until it is cleared by a clear timer interrupt signal. The output of the latch 98 is logically combined by an AND gate 100 with the instruction signal asserted at step 94. When the latch is set and the instruction signal is asserted, then an interrupt is generated as the output of the AND gate 100 and is used to trigger an interrupt that performs scheduling operations using the interrupt processing mechanisms provided within the system for standard interrupt processing. Once the interrupt signal has been generated, this in turn triggers the production of a clear timer interrupt signal that clears the latch 98 until the next timer output pulse occurs.

[0091]FIG. 13 is a signal diagram illustrating the operation of the circuit of FIG. 12. The processor core clock signals occur at a regular frequency. The timer 96 generates timer signals at predetermined periods to indicate that, when safe, a scheduling operation should be initiated. The timer signals are latched. Instruction signals are generated at times spaced apart by intervals that depend upon how quickly a particular Java bytecode was executed. A simple Java bytecode may execute in a single processor core clock cycle, or more typically two or three, whereas a complex Java bytecode providing a high level management type function may take several hundred processor clock cycles before its execution is completed by the software interpreter. In either case, a pending asserted latched timer signal is not acted upon to trigger a scheduling operation until the instruction signal issues indicating that it is safe for the scheduling operation to commence. The simultaneous occurrence of a latched timer signal and the instruction signal triggers the generation of an interrupt signal followed immediately thereafter by a clear signal that clears the latch 98.

[0092] Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims. We Claim 

1. Apparatus for processing data, said apparatus comprising: (i) a processor core operable to execute operations as specified by instructions of a first instruction set; (ii) an instruction translator operable to translate instructions of a second instruction set into translator output signals corresponding to instructions of said first instruction set, at least one instruction of said second instruction set specifying an operation to be executed using one or more input variables; (iii) an interrupt handler responsive to an interrupt signal to interrupt execution of operations corresponding to instructions of said first instruction set after completion of execution of a currently executing operation; and (iv) restart logic for restarting execution after said interrupt; wherein (v) said instruction translator is operable to generate a sequence of one or more sets of translator output signals corresponding to instructions of said first instruction set to represent said at least one instruction of said second instruction set, each sequence being such that no change is made to said one or more input variables until a final operation within said sequence is executed; and (vi) after occurrence of an interrupt during execution of a sequence of operations representing said at least one instruction of said second instruction set: (a) if said interrupt occurred prior to starting execution of a final operation in said sequence, then said restart logic restarts execution at a first operation in said sequence; and (b) if said interrupt occurred after starting execution of a final operation in said sequence, then said restart logic restarts execution at a next instruction following said sequence.
 2. Apparatus as claimed in claim 1, wherein said translator output signals include signals forming an instruction of said first instruction set.
 3. Apparatus as claimed in any one of claims 1 and 2, wherein said translator output signals include control signals that control operation of said processor core and match control signals produced on decoding instructions of said first instruction set.
 4. Apparatus as claimed in any one of claims 1, 2 and 3, wherein said translator output signals include control signals that control operation of said processor core and specify parameters not specified by control signals produced on decoding instructions of said first instruction set.
 5. Apparatus as claimed in any one of the preceding claims, wherein said restart logic is part of said instruction translator.
 6. Apparatus as claimed in any one of the preceding claims, wherein said restart logic stores a pointer to a restart location within instructions of said second instruction set that are being translated, said pointer being advanced upon execution of said final operation.
 7. Apparatus as claimed in claim 6, wherein said pointer is a program counter value pointing to a memory address of a memory location storing an instruction of said second instruction set currently being translated.
 8. Apparatus as claimed in any one of the preceding claims, wherein instructions of said second instruction set specify operations to be executed upon stack operands held in a stack and said input variables include input stack operands.
 9. Apparatus as claimed in claim 8, wherein any stack operands removed from said stack by execution of said at least one instruction of said second instruction set are not removed until after execution of said final operation has commenced.
 10. Apparatus as claimed in any one of claims 8 and 9, wherein any stack operands added to said stack by execution of said at least one instruction of said second instruction are not added until after execution of said final operation has commenced.
 11. Apparatus as claimed in any one of the preceding claims, wherein said input variables include system state variables not specified within said second instruction.
 12. Apparatus as claimed in any one of the preceding claims, wherein said processor has a register bank containing a plurality of registers and instructions of said first instruction set execute operations upon register operands held in said registers.
 13. Apparatus as claimed in claim 12, wherein a set of registers within said register bank hold stack operands from a top potion of said stack.
 14. Apparatus as claimed in claim 13, wherein said instruction translator has a plurality of mapping states in which different registers within said set of registers hold respective stack operands from different positions within said stack, said instruction translator being operable to move between mapping states when said final operation is executed so as to update said input variables.
 15. Apparatus as claimed in any one of the preceding claims, wherein said instructions of said second instruction set are Java Virtual Machine instructions.
 16. A method of processing data, said method comprising the steps of: (i) executing operations as specified by instructions of a first instruction set; (ii) translating instructions of a second instruction set into translator output signals corresponding to instructions of said first instruction set, at least one instruction of said second instruction set specifying an operation to be executed using one or more input variables; (iii) in response to an interrupt signal, interrupting execution of operations corresponding to instructions of said first instruction set after completion of execution of a currently executing operation; and (iv) restarting execution after said interrupt; wherein (v) said step of translating generates a sequence of one or more sets of translator output signals corresponding to instructions of said first instruction set to represent said at least one instruction of said second instruction set, each sequence being such that no change is made to said one or more input variables until a final operation within said sequence is executed; and (vi) after occurrence of an interrupt during execution of a sequence of operations representing said at least one instruction of said second instruction set: (a) if said interrupt occurred prior to starting execution of a final operation in said sequence, then restarting execution at a first operation in said sequence; and (b) if said interrupt occurred after starting execution of a final operation in said sequence, then restarting execution at a next instruction following said sequence.
 17. A computer program product holding a computer program for controlling a computer to perform the method of claim
 15. 18. Apparatus for data processing substantially as hereinbefore described with reference to the accompanying drawings.
 19. A method of data processing substantially as hereinbefore described with reference to the accompanying drawings.
 20. A computer program product holding a computer program for controlling a computer to perform a method substantially as hereinbefore described with reference to the accompanying drawings. 